Parkinson's disease is a disabling, progressive illness that affects one in 1,000 people and generally occurs in people over the age of 50 years. Patients with Parkinson's disease have a deficiency of the neurotransmitter dopamine in the brain as a result of nigrostriatal pathway disruption caused by degeneration of the substantia nigra. Levodopa (L-dopa or L-3,4-dihydroxyphenylalanine), an immediate precursor of dopamine, is the most commonly prescribed drug for treatment of this disease.
Following oral administration, levodopa is rapidly absorbed via an amino acid transporter present in the upper small intestine. Due to the narrow distribution of this transporter system, the window available for levodopa absorption is limited and the extent of absorption can depend on the rate at which the drug passes through the upper gastrointestinal tract.
Intestinal metabolism of levodopa is the major source of first pass loss of the drug. Approximately 35% of an administered dose of levodopa reaches the systemic circulation as intact levodopa after oral administration in patients (Sasahara, J. Pharm. Sci 1990, 69, 261). Once absorbed, levodopa is rapidly metabolized to dopamine by L-aromatic amino acid decarboxylase (AADC) enzymes in the peripheral tissues (e.g., intestines and liver). For this reason, levodopa is normally co-administered with a decarboxylase enzyme inhibitor such as carbidopa or benserazide. When administered with carbidopa, the plasma concentration of intact levodopa increases and thus more levodopa becomes available to be transported into the central nervous system where it is converted to dopamine. Carbidopa and benserazide do not cross the blood-brain barrier to a significant extent and therefore do not inhibit the required conversion of levodopa to dopamine in the brain.
The use of prodrugs of levodopa to improve the pharmacokinetics of levodopa has been proposed. Many of these prodrugs are simple esters of levodopa (see, e.g., Chiesi, U.S. Pat. Nos. 5,017,607 and 4,826,875; Repta, U.S. Pat. Nos. 4,873,263, 4,771,073 and 4,663,349; Bodor et al., U.S. Pat. No. 4,311,706; Konishi and Ienaga, Japanese Patent No. JP58024547; Juncos et al., Neurology 1987, 37, 1242; and Cooper et al., J. Pharm. Pharmacol. 1987, 39, 627-635). An oral formulation of levodopa methyl ester (Levomet®, CHF 1301) has been described (Chiesi Pharmaceuticals). The ethyl ester of levodopa (TV-1203) is under clinical investigation as a potential therapy for Parkinson's disease when co-administered with carbidopa (Milman et al., U.S. Pat. No. 5,607,969). A sustained release cellulose formulation of levodopa ethyl ester in a mixture of hydroxypropylmethyl cellulose, hydroxypropyl cellulose, and a carboxyvinyl polymer has also been described (Cohen et al., U.S. Pat. No. 5,840,756). However, oral administration of this formulation to healthy adults pretreated with carbidopa produced a plasma levodopa terminal half-life of only 2 hours, comparable to that of Sinemet® CR.
A pivaloyl ester of levodopa (NB-355) has been described (Tsuchiya and Hayashi, European Patent No. 0 309 827). Following oral administration of NB-355, a rapid increase in the plasma concentration or in the elimination of levodopa was not observed and the duration of circulating levodopa was prolonged, although plasma concentrations of levodopa were low. The potential for using ester prodrugs of levodopa to enhance rectal absorption of the drug has also been described (Repta, U.S. Pat. Nos. 4,663,349, 4,771,073, and 4,873,263). Notably, the absorption of simple alkyl esters of levodopa has been shown to be greater following rectal absorption than following oral dosing (Fix, et al., Pharm. Res. 1989, 6, 501-5; and Fix, et al., Pharm. Res. 1990, 4, 384-7). This effect is attributed to the decreased abundance of esterases in the large intestine relative to the small intestine. Therefore, selective delivery of a prodrug of levodopa to the large intestine in a sustained release formulation might be expected to provide a greater oral bioavailability and a prolonged systemic exposure to the drug. A series of glycolic acid ester containing prodrugs of levodopa has been described (Wermuth, U.S. Pat. No. 4,134,991). Lipid conjugates of levodopa to facilitate the entry of levodopa into cells and tissues have also been described (Yatvin, U.S. Pat. No. 5,827,819). Glyercol derivatives of levodopa are described by Dumont, International Application Publication No. WO 86/04579. Catechol protected levodopa derivatives intended to provide sustained clinically effective blood concentrations of levodopa are disclosed in Tsuchiya et al., U.S. Pat. No. 4,966,915.
Thus, the development of levodopa prodrugs that can be efficiently absorbed throughout the gastrointestinal tract, including the colon, and reduce first-pass metabolism of levodopa is highly desirable.
The human gastrointestinal tract includes the small intestine and the large intestine. The human small intestine is a convoluted tube about twenty feet in length between the stomach and large intestine. The small intestine is subdivided into the duodenum, the jejunum, and the ileum. The large intestine is about 5 feet in length and runs from the ileum to the anus. The large intestine is divided into the caecum, colon, and the rectum. The colon is divided into four parts including the ascending, traverse, descending, and the sigmoid flexure. In general, an orally ingested compound resides about 1 to 6 hours in the stomach, about 2 to 7 hours in the small intestine, and about 8 to 18 hours in the colon. Thus, the greatest period of time for sustained release of a compound occurs when the compound is passing through the colon.
Certain active transporter proteins are known to be expressed throughout the gastrointestinal tract. An active transporter refers to a membrane-bound protein that recognizes a substrate and affects the entry of the substrate into or exit from a cell by carrier-mediated transport or receptor-mediated transport. Active transport includes movement of molecules across cellular membranes that is directly or indirectly dependent on an energy mediated process, such as for example by a process driven by ATP hydrolysis, or by an ion gradient, which occurs by facilitated diffusion mediated by interaction with specific transporter proteins through a modulated solute channel. Examples of solute mediated transporters include organic cation transporters such as OCTN1 and OCTN2, which are expressed in the epithelial cells lining a human colon as well as in the small intestine.
More recently, levodopa prodrugs designed to be absorbed in both the small and large intestines have been described in Xiang et al., U.S. Application Publication Nos. 2005/0282891 and 2006/0020028, each of which is incorporated by reference herein in its entirety. These levodopa prodrugs can achieve a bioavailability of levodopa that is at least two times greater than the bioavailability of levodopa when intracolonically administered on an equivalent molar basis. The mesylate salt of one of these prodrugs, (2R)-2-phenylcarbonyloxypropyl (2S)-2-amino-3-(3,4-dihydroxyphenyl)propanoate mesylate, and crystalline forms thereof are described in Xiang et al., U.S. Application Publication No. 2007/0225366. The prodrugs described by Xiang et al. can be efficaciously incorporated into sustained release formulations including osmotic delivery devices to provide sustained systemic exposure to levodopa upon oral administration to a patient.